1. Field of the Invention
The present invention relates to a thermosetting resin composition comprising fine particles of an elastomer component homogeneously dispersed in a thermosetting resin.
2. Description of the Background Art
A disadvantage of thermosetting resins such as phenol resins, epoxy resins, melamine resins, urea resins, polyimide resins, and the like is their brittleness which offsets their excellent mechanical strength. A strong desire therefore existed for the development of molded articles of thermosetting resins with greater impact resistance and adequate flexibility.
One proposal for providing thermosetting resins with more flexibility is blending rubber with them. A known example of such a mixture of rubber and a thermosetting resin is a mixture of butadiene-acrylonitrile copolymer and phenol resin. The use of rubbers with functional groups in order to provide reactivity with epoxy resins has been studied and reported by a number of publications, e.g., Japanese Patent Publication No. 30133/1982. The studies heretofore undertaken, however, dealt with compositions containing thermosetting resins and rubbers having polarities close to each other and thus having a greater mutual solubilities. Products with satisfactory mechanical strength can be obtained by blending a thermosetting resin and a rubber having a good mutual solubility. Such products, however, have a defect in their decreased fluidity, which impairs moldability of the thermosetting resin compositions. Another problem is that the mixture must be blended with a great care so as to ensure adequate dispersion of rubbers throughout thermosetting resins.
In order to improve the dispersibility of rubbers in thermosetting resin compositions the present inventors previously proposed the use of rubbers crosslinked by polyfunctional monomers (Japanese Patent Laid-open (ko-kai) No. 228562/1985). This method, however, could not improve the fluidity of thermosetting resin compositions.
Japanese Patent Laid-open (ko-kai) No. 22850/1987 discloses a thermosetting resin composition comprising an epoxy resin and fine particles of rubber covered with a resin having a glass transition point above room temperature. Since such resins with a glass transition point above room temperature do not contribute to the improvement in the impact strength, the resulting thermosetting resin composition possesses insufficient mechanical strength and rigidity.
Japanese Patent Laid-open (ko-kai) No. 53370/1987, on the other hand, discloses a process of dispersing rubbery particles in epoxy resin by polymerizing monomers which produces a polymer having a glass transition point lower than room temperature in the epoxy resin. The problem of this process is in the difficulty in controlling particle sizes of the rubbery particles with acceptable reproducibility because the sizes of rubbery particles are greatly affected by the stirring conditions, the temperature, and the like in the polymerization reaction.
Japanese Patent Laid-open (ko-kai) No. 91755/1983 discloses a process of producing an epoxy resin composition into which vulcanized rubber with a particle size of 0.5-30 .mu.m is dispersed. In the process, a liquid rubber, which is insoluble in the epoxy resin, is first dispersed in the epoxy resin and then vulcanized. Controlling the rubber particle size with good reproducibility is, however, extremely difficult in this process. In addition, the process can only produce rubbery particles with a very wide particle size distribution.
U.S. Pat. No. 4,778,851 discloses a process for dispersing graft rubber particles in an epoxy resin. Dispersing graft rubber particles homogeneously in epoxy resin requires that the graft rubber particles be dispersed in water or dissolved in a solvent. The process does not provide epoxy resin compositions with sufficiently improved properties.
The present inventors have undertaken extensive studies in order to obtain a rubber-thermosetting resin composition without drawbacks mentioned above; a composition in which rubber particles with a uniform particle size are dispersed in a thermosetting resin and which exhibits superior flexibility, mechanical properties, fluidity, and moldability at the same time. As a result, the present inventors have found that a specific rubbery copolymer can satisfy these requirements when blended with a thermosetting resin in a certain proportion.